1. Field of the Invention
The present invention relates to a method of preforming a friction material used as, for example, a disk brake friction pad for use in an automobile.
2. Related Art
For example, a disk brake friction pad for use in an automobile is manufactured by the steps of introducing a friction material matrix which consists of a mixture of a fibrous base material made from, e.g., metallic, inorganic, or organic fibers, a binder made from a thermosetting resin powder, and various fillers, e.g., a friction adjustor and a lubricant, into a molding die; stamping the friction material matrix at room temperature; preforming the stamped friction material matrix; transferring the thus formed preform to a thermoforming die having a back plate set thereto; and heating the thermoforming die under pressure (hereinafter referred to as thermoforming).
In the above described manufacturing processes, the friction material matrix introduced into the molding die is leveled before being preformed. For example, Japanese Patent Publn. No. Hei-2(1990)-51719 discloses a method of forming a two-layer disk brake friction pad. Specifically, the disk brake friction pad comprises a brake lining layer made from semi-metallic material, and a heat insulating layer which is made from material with plenty of heat insulating properties and has a projecting boss to be jointed into a dovetailed hole formed in a back plate. The method is characterized by comprising the steps of introducing material for use as the heat insulating layer into a molding die, forming the boss by stamping the heat insulating layer by lowering a boss-stamping rod, leveling the top of the heat insulating layer, introducing a powder and granular material, such as, a semi-metallic material on the flat top of the heat insulating layer, further leveling top of the semi-metallic material, and ramming the semi-metallic material and the heat-insulating layer by lowering a pressing ram into the molding die. Further, Japanese Patent Publn. No. Hei-3(1991)-6878 discloses a flattening apparatus preferably for use in the above described flattening operation.
As a result of the surface of the friction material matrix being leveled through the flattening operation, the thickness of the friction material matrix in the molding die becomes even. The distribution of pressure of the pressing ram also becomes even, which improves the moldability of the friction material matrix.
The friction material matrix consists of a mixture of a fibrous base material, a binder, and various fillers, and these materials are solid. When the friction material matrix is introduced into the molding die, a plenty of air is mixedly trapped in the friction material matrix. Some of the air escapes through voids between the interior wall of the molding die and the friction material matrix associated with a compressing operation. However, much of the air still remains in the preform. If the preform containing the residual air is thermally formed, the residual air expands, so that holes are formed in a resultant molding. Eventually, cracks or bulges are produced in the molding, that is, a disk brake friction pad.
The previously described flattening operation is only intended to level the upper surface of the friction material matrix. By means of this flatting operation, the adhesion between the two materials contained in the two-layer disk brake friction pad, as disclosed in Japanese Patent Publn. No. Hei-2(1990)-51719, and the adhesion of a contact surface between the friction material matrix and the pressing ram are improved. However, it is impossible to eliminate the air still remaining in the friction material matrix by the flattening operation.
In view of the friction effect and mechanical strength of the disk brake friction pad, it is desirable that the fibrous base material, the binder, and the various fillers should become united to each other, specifically, fibers should become closely interlocked together, and they should become integrally united together while the fillers run deep into the fibrous base material, in addition to the even distribution of the fibrous base material, the binder, and the fillers within the molding. The friction material matrix is prepared by formulating predetermined amounts of solid materials, that is, the fibrous base material, the binder, and the fillers, and by simply kneading them with each other. As a result, the materials become locally uneven and less united together.
However, it is impossible to make the materials united together as well as to make the friction material matrix homogenous to a much greater extent by use of the previously described flattening operation.